Wow, will someone kick me in the chest to get my heart started again? I use two 12V chargers, one for each battery. If one takes longer to charge after they've been sitting for a while, I know I need to go get the load-tester for that battery. My two trickle chargers are about $25 each, besides one is perfect for charging the generator battery after it's been sitting for a while.

We use many of those 2.5 Amp, 24 VDC Battery Tender, water proof chargers in out business, also the similar model for 12 VDC @ 5 Amp, they have a good track record for working, not cheap. Over the years have been thru many versions of chargers, in general, the Northern Hydraulic & Harbor Freight chargers have proven to be very costly as they are not dependable. Cummins, Cat & Kohler all have good chargers, but they are also very costly, one model cost more to repair than buying a new identical one, go figure that one.When you get to the 32 V, 36V and 48 V stuff, they can get real costly.

I was in error when I posted that charger. The one I use is here, although it has a yellow case instead of red. I use it to maintain all my different batteries at different times, mainly to maintain and desulfate them. They say they have one that will work on 24 volt pairs, but not on a single batt; see info.

Spring for the Battery Minder 24041 cost almost twice as much as the Battery Tender but a far superior unit IMO Cat dealers also sell their version of the PluseTech in 24v sometimes they are on sell this time of the year

I'm just using my Magnum 4024 to maintain the batteries this winter, with the start batteries bridged in to the house set. So far when I've looked it's trickle charging at 0 amps, which means it's off and waiting for the battery voltage to fall to start trickle charging again. Which is what it's supposed to do, as I understand it. One reason I picked that unit was it had this "off and waiting" charge mode, which is almost a fifth stage. I'll probably equalize them and check the water level around New Years. Mind you, it's a bloody expensive trickle charger, but it has other uses...